Numerous research and development efforts have so far been made on thermoplastic resin film by many researchers, enterprises and the like. Thermoplastic resin film, although its raw material is relatively inexpensive, is excellent in mechanical property, chemical resistance, transparency, water vapor permeability and so on, and is therefore used in variety fields such as packaging, general merchandise, agriculture, industry, food, and medical care.
In recent years, there have appeared many examples of using thermoplastic resin film in the optical field. Thermoplastic resins (e.g. polycarbonate and cyclic polyolefin) have a relatively good light transmittance, and may be given optical anisotropy (orientation) by stretching treatment (uniaxial stretching or biaxial stretching). Film produced from such thermoplastic resin given an orientation property may be conveniently used as retardation film for liquid crystal displays (LCDs) and the like.
Various methods of manufacturing such thermoplastic resin film are known and have been implemented. The thermoplastic resin film manufacturing methods generally used in industry include a solvent casting method that forms film by casting, to the glass plate or the like, a resin solution having a resin dissolved in a solvent (see Patent Application “Kokai” No. 5-239229, for example), a T-die extrusion method that forms film by cooling with a chill roll a melted resin extruded from an extruder (see Patent Application “Kokai” No. 2000-219752, for example), a tubular extrusion method that extrudes a melted resin in a tubular form from an extruder (see Patent Application “Kokai” No. 59-120428, for example), and a blown film extrusion method that shapes a resin while applying an air pressure inside the resin extruded in a tubular form (see Patent Applications “Kokai” No. 60-259430 and No. 8-267571, for example).
However, the conventional thermoplastic resin film manufacturing methods noted above have various problems. The solvent casting method, for example, has a drawback of requiring a large apparatus as a whole since a solvent is used, and this results in an increased manufacturing cost. As a more serious problem, the solvent casting method uses a large quantity of solvent, imposing a great load on environment, which is against today's current of environmental protection.
The T-die extrusion method also has a problem of requiring a large apparatus which needs a large installation area, and moreover, the apparatus itself is very expensive. A further problem of the T-die extrusion method is that, when an attempt is made to reduce film thickness, the thickness accuracy of film ends will become low, and the film ends must be discarded. This results in a reduced product yield.
Generally, the film produced by the T-die extrusion method is stretched in a tentering mode. In the tentering mode, end regions of the film are pinched with clips. Thus, only the film central part could be used because of large variations in the slow axis angle in the end regions.
On the other hand, the tubular extrusion method allows equipment to be relatively small, and its product yield is also good. Thus, this method is more widely used in the field of resin film molding than before. The tubular extrusion method can obtain resin film in a tubular shape, and this tubular resin film may be cut open in the longitudinal direction with a cutting device such as a roll cutter, to obtain a broad resin film. With such conventional tubular extrusion method, however, it has been very difficult to obtain resin film of fixed quality on a regular basis. A resin extruded in a tubular form from an extruder is unstable and vulnerable to the influence of outside environment, and its shape can change easily. With the tubular extrusion method, therefore, it has been almost impossible to manufacture steadily resin film products usable as retardation film or the like, having a small and uniform film thickness, and having smooth surfaces.
The blown film extrusion method is a method that, after extruding a melted resin in a tubular form from an extruder, shapes the resin film while blowing air inside the resin. With this method, as with the above-noted methods, the instability of the resin extruded in a tubular form from the extruder readily results in creases, slacks, lenticulations and the like on the film due to minor changes in film tension and turbulences of air currents. Thus, with the blown film extrusion method also, the problem remains to be solved that it is difficult to manufacture steadily resin film products having a small and uniform film thickness, and having smooth surfaces.
The film produced by the conventional tubular extrusion method or blown film extrusion method has large thickness variations, and could not be used conveniently as retardation film or the like.
Therefore, this invention has been made having regard to the problems noted above, and its object is to provide a manufacturing apparatus for manufacturing a tubular resin film of high quality and little thickness variations, which is stretched and given an orientation, and suitable as retardation film or the like.